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An indirect phenotyping method was developed in order to estimate the susceptibility of rubber tree clonal varieties to Corynespora Leaf Fall (CLF) disease caused by the ascomycete Corynespora cassiicola. This method consists in quantifying the impact of fungal exudates on detached leaves by measuring the induced electrolyte leakage (EL%). The tested exudates were either crude culture filtrates from diverse C. cassiicola isolates or the purified cassiicolin (Cas1), a small secreted effector protein produced by the aggressive isolate CCP. The test was found to be quantitative, with the EL% response proportional to toxin concentration. For eight clones tested with two aggressive isolates, the EL% response to the filtrates positively correlated to the response induced by conidial inoculation. The toxicity test applied to 18 clones using 13 toxinic treatments evidenced an important variability among clones and treatments, with a significant additional clone x treatment interaction effect. A genetic linkage map was built using 306 microsatellite markers, from the F1 population of the PB260 x RRIM600 family. Phenotyping of the population for sensitivity to the purified Cas1 effector and to culture filtrates from seven C. cassiicola isolates revealed a polygenic determinism, with six QTL detected on five chromosomes and percentages of explained phenotypic variance varying from 11 to 17%. Two common QTL were identified for the CCP filtrate and the purified cassiicolin, suggesting that Cas1 may be the main effector of CCP filtrate toxicity. The CCP filtrate clearly contrasted with all other filtrates. The toxicity test based on Electrolyte Leakage Measurement offers the opportunity to assess the sensitivity of rubber genotypes to C. cassiicola exudates or purified effectors for genetic investigations and early selection, without risk of spreading the fungus in plantations. However, the power of this test for predicting field susceptibility of rubber clones to CLF will have to be further investigated.

No abstract

Black pepper is an industrial crop with high economic and export value. However, black pepper production in Vietnam has been seriously affected by the root-knot nematodes, Meloidogyne spp. The purpose of this study was to select active endophytic bacteria (EB) for the cost-effective and environmentally friendly management of Meloidogyne sp. Thirty-four EB strains were isolated. Of these, five isolates displayed the highest activity, demonstrating 100% mortality of J2 nematodes. These active EB were identified based on sequencing and phylogenetic analysis of the 16S rRNA gene; notably, all the potential endophytic bacterial strains belong to the genus of Bacillus. In greenhouse tests, Bacillus megaterium DS9 significantly reduced nematodes in the soil and pepper plant roots with great inhibition values of 81.86% and 73.11%, respectively, with the lowest rate of nematodes built up at 0.23. This active antinematodes strain also showed good effect on promoting pepper plant growth. Some enzymatic activities, including chitinase and protease activity related to the biocontrol of Meiloidogyne sp., were also detected. The results investigated in the current study suggested that these selected EB strains may be good candidates for biocontrol agents of Meloidogyne sp., and plant promoting effects. The results also enhanced the novel active antinematode endophytic bacterial communities.